1. Field of the Invention
This invention relates to the control of the operation of a seismic signal transducer by wireless trnsmission of a coded signal.
2. Description of the Prior Art
In one method of seismic exploration, one or more mobile vibratory-type seismic signal transducers inject a sweep signal into the ground. A plurality of seismic sensors, connected to a recording vehicle through a multiconductor cable, detect the sweep signals after they have been reflected from subsurface earth layers. The detected signals are transmitted to the recording vehicle through the multiconductor cable.
In a typical operation, several hundred sensors are laid out along a segment of the line of survey. The line segment or "spread" may extend for 2 or 3 miles. The recordng vehicle, to which the sensors are connected through the multiconductor cable, is positioned at a convenient location along the spread. One or more mobile seismic signal transducer units such as vibrators proceed along the spread, stopping at selected stations to transmit a seismic signal into the ground. Commonly, the spacing between stations is 200-300 feet. Each station is occupied by the moble units for about 10-20 seconds. Although the line of station locations parallels the line of survey, terrain conditions and obstructions occasionally force the mobile units to detour some distance away from the actual seismic sensor spread.
In operation, the recording truck issues a START-SWEEP command to the mobile units to generate a sweep and, at the same time, activates the recording system to record the reflected signals detected by the seismic sensors. An accelerometer is attached to the vibrating transducer on the mobile unit to record the sweep signal actually injected into the ground. Upon command it transmits this data back to the recording truck as a reference sweep for verification that the correct sweep was in fact generated.
The START-SWEEP command and the reference sweep could be directly transmitted between the mobile units and the recording truck by wire land line, such as through the multiconductor cable. Such a procedure is impractical, however, since a physical wire-connection between the mobile units and the cable would have to be made at every station. Because the mobile units linger but a few tens of seconds at each station, there is no time to make a physical wired connection to the recording vehicle. Accordingly, the signals are customarily transmitted between recording truck and mobile units by a VHF radio link.
The START-SWEEP radio command is a 128-millisecond burst of coded bits, transmitted as an FM signal over the VHF channel. The returned reference sweep signal, of some 15 seconds duration, is a sub-carrier FM signal transmited over the same channel. Over a communications channel, the START-SWEEP command sounds like a harsh "zip." The reference sweep signal is heard as a chirp signal of rising or falling pitch. In an actual seismic operation, these signals are transmitted every 20-30 seconds.
The number of VHF channels allotted to the petroleum industry in general is limited; to geophysical field crews, only one or two such channels are available In a high-traffic area, voice communications are virtually impossible over a channel that is also used for vibrator operations. An alternate radio link between recorder and mobile units that will not interfere with normal voice traffic is therefore highly desirable.